Intrauterine exposure to gestational diabetes (GDM) is associated with a high risk of childhood obesity and explains over 47% of pediatric type 2 diabetes. To mitigate this impact of GDM on children, high-risk pregnancies must be identified earlier and effective interventions must be developed. Maternal obesity, and especially central obesity, is a known predictor of GDM and research shows that exercise is protective. What remains unknown are the roles of sedentariness and cardiorespiratory fitness (CRF) within the obesity-GDM relationship. The purpose of the proposed study is to determine the independent and interaction effects among ultrasound measures of abdominal adiposity, sedentariness, and CRF on insulin sensitivity and pancreatic ?-cell responsivity in pregnancy. Once these relationships are delineated, then effective lifestyle interventions can be developed. This is a prospective cohort study of 115 racially diverse, nulliparous pregnant women,18-35 years old, and overweight or obese (BMI 25-35) with no history of metabolic disease. Measures will include body composition, including ultrasound assessment of abdominal adipose tissue as intra-abdominal and subcutaneous, sedentary behavior via 4-day accelerometry, minimal model computation of insulin sensitivity and pancreatic ?-cell responsivity from 2-hour frequently sampled oral glucose tolerance test, and CRF via graded exercise test. Four visits will take place in the Kennesaw State University Exercise Science (ES) Physiology Laboratory at baseline (12-15 weeks), mid-gestation (18-19 weeks), third trimester (26-28 weeks), and post-partum (4 weeks). Independent and interaction effects of abdominal adipose tissue thickness, sedentary behavior, and CRF on gestational insulin sensitivity and pancreatic ?-cell responsivity will be tested.